Yes it is C but for future reference, both A and B are referring to solids, and D is referring to a gas, hope this is helpful later!
Answer:
Mold fossils A fossilized impression made in the substrate; a negative image of the organism.
Cast fossils Formed when a mold is filled in.
Trace fossils or Ichnofossils Fossilized nests, gastroliths, burrows, footprints, etc.
True form fossils Fossils of the actual animal or animal part.
I think the first one is the the forest industries keeps changing, so that is why the economics delayed the scientists.
